Consequences of early brain lesions in the Syrian hamster are being studied. The approach is designed to exploit the exceptionally clear and contrasting behavioral effects of visual-cortex lesions (trouble with pattern discrimination) and midbrain tectum lesions (trouble with visually guided orientation) in the adult Syrian hamster. If either type of lesion is made in the neonate rather than in the adult animal, the behavioral defect, examined at maturity, is often much less severe or even absent. In cases of early lesions of the midbrain tectum, specific anomalies in the direction and the form of visually elicited turning movements have also been discovered. Anomalous growth of optic-tract connections, particularly in the midbrain, appears to explain, at least partially, the spared or altered functions. The separate factors underlying the growth of altered connections are being analysed. Structural studies include an analysis of topography of retinal projections in normal animals and in animals with partial lesions of tectum or retina at birth, and examination of neocortical projections in similar animals. Functional studies include detailed descriptions of visually elicited turning movements in normal hamsters and in hamsters with abnormal retinal projections.